An important step in the manufacture of a modern electronic device is putting together printed wiring assemblies that carry components integral to the device. Each printed wiring assembly, alternatively called a module, consists of a printed wiring board with one or more of the electronic components that form the completely assembled electronic device mounted thereon. The printed wiring board is a rigid planar sheet of material with conductive traces formed thereon that provide separate conductive paths between the board components. An assembled printed wiring assembly is at least part of an electronic circuit that forms at least part of the electronic device. Usually, a modern electronic device, such as a computer or other processing system, includes one or more printed wire assemblies that carry thereon individual components that form the assembled device.
Traditionally, lead-in-hole connections have been used to mount electronic components onto printed wiring boards. These connections comprise a set of reinforced conductive leads or pins extending from the component that are inserted into plated holes on the printed wiring board. Solder, or a friction fit socket is used to secure the leads in the holes and provide an electrical connection to the conductors on the board.
However, frequently traditional lead-in-hole connections cannot be used to provide the connections required by modern electronic devices. This is because modern electronic devices generally include one or more integrated components that are mounted to multi-layer printed wiring boards. Each component may comprise one or more semiconducting components, known as "chips" that are packaged together in a carrier. Each chip comprises a large number of individual components fabricated together on a single, small section of semiconducting material to form an integrated circuit. A multi-layer printed circuit board comprises a number of individual layers, each with conductors thereon, that are stacked and pressed together to form a single wiring board. Multi-layer circuit boards provide most of the conductors that are needed to connect individual chips mounted thereon together to form a single integral circuit. The use of integrated components and multi-layer printed wiring boards has contributed to making modern electronic devices more powerful and smaller than their predecessors.
There are a number of reasons why it has proven impractical to use traditional mounting methods to attach some components to multi-layer printed wiring boards. Many assembled components include a large number of leads that provide the signal and power connections required for their operation. Providing a sufficient number of reinforced leads for each of the required connections significantly increases the overall size of a components, defeating the advantage of its miniaturization. Also, a significant amount of area would be taken up on the printed wiring board the components are mounted on by having to provide the plated holes required for all of the leads. Moreover, providing mounting holes on a multi-layer printed circuit board is further complicated because the conductors on each layer of the board must be designed around the holes which would extend therethrough. In other words, providing a printed wiring board with mounting holes restricts the paths of the conductors that can be formed thereon. This increases the difficulty of designing board conductors required for the specific circuit.
As a consequence of the problems and limitations associated with traditional lead-in-hole mounting, surface mounting of components to printed wiring boards has become a popular alternative. Surface mounting involves using solder to bond component leads to conductive contact pads on the surface of a printed wiring board. The surface contact pads are connected to conductors located on the board, or located within it in the case of multi-layer printed wiring boards. The solder used to provide the connection has sufficient strength to secure the components, leads to the contact pads on the surface of the board, and is sufficiently conductive to provide an electrical path therebetween.
Surface mounting does not require the large reinforced leads of lead-in-hole mounting. Nor does it require plated holes that extend through a printed wiring board that conductors must be designed around. Thus, surface mounting has become a popular method of attaching electrical components to printed wiring boards they are associated with.
An important consideration when surface mounting components is the need to initially apply the correct amount of solder to the board contact pads before the actual lead-to-contact pad bonding process. If too little solder is applied, it may be insufficient to secure the component to the contact pad. Consequently, over time a particular component attached to a printed wiring board may break off, rendering the entire circuit inoperable. On the other hand, if too much solder is applied, when the solder is heated during the bonding process, it may flow off the surface contact pad it is on and on to an adjacent contact pad. The solder, being conductive, would thus create an undesirable conductive path between the contact pads. Thus, the amount of solder that must be applied prior to the bonding must be precisely controlled.
One method of controlling the amount of solder applied during surface mounting is to apply the solder, in paste form, with a stencil. A metal stencil, with some flexibility, is placed a small distance above the printed wiring board that the components are to be mounted to. Formed in the stencil are openings that are in registration over the surface contact pads. Solder paste is then spread over the top of the stencil and pressed into the openings. A squeegee is then pressed down over the exposed surface of the stencil. The squeegee forces the adjacent portion of the stencil against the printed wiring board, and the solder paste through the openings. After the squeegee passes over the stencil portion, the stencil "snaps back" to its original location, leaving the appropriate amounts of solder on each of the contact pads. The components may then be bonded to the printed wiring board. This method of applying solder, and a discussion of surface mounting, is described in Mullen, How to Use Surface Mount Technology, incorporated herein by reference.
Recent advances in semiconductor manufacturing has made the fabrication of large scale integration (LSI) and very large scale integration (VLSI) chips possible. LSI and VLSI chips contain thousands to hundred of thousands of individual components per chip. LSI and VLSI chips are more compact than their predecessors, and can perform their intended functions more efficiently. Present and future advances in component manufacturing continue to make most electronic devices more powerful, and more miniaturized, than their forerunners.
To date, though, it has been difficult to apply the correct amount of solder to printed wiring board needed to bond LSI and VLSI components. This is because each of these components contain a large number of leads that are necessary for all the input and output signals these chips need and generate in the course of their operation. Components comprising LSI and VLSI chips with one hundred or more leads are not uncommon. These leads are spaced a small distance apart from each other, in other words, the between lead pitch is very fine. The leads are spaced close together at a fine pitch to minimize the overall size of the component package. Thus, only a small amount of solder is required to surface mount the small, fine pitch leads of LSI and VLSI components.
The problem of applying solder to contact pads for surface mounting of fine pitch leads is further complicated by the need to simultaneously apply larger amounts of solder required to mount standard leads found on other components. This is because most printed wiring assemblies carry components that have fine pitch leads, such as components comprising LSI and VLSI chips, also carry other components that have standard pitch leads, such as packaged conventional chips and discrete components such as resistors, capacitors and diodes.
Thus far, it has been difficult to apply to a printed wiring board the relatively large amounts of solder needed for standard lead bonding, and the smaller amounts of solder needed for fine pitch lead bonding. This is because the techniques used to apply one amount of solder do not work for applying the other amount of solder. As a result, various processes are being used to apply all the required solder with only limited degrees of success.
One such process is to use a single stencil with both large, widely spaced openings for the solder needed for standard lead bonding, and smaller, closely spaced openings needed for fine pitch lead bonding. The correct amount of solder needed for all the leads can often be applied by this process. However, this method has a significant failure rate since often a large number of boards with the incorrect amount of solder thereon are produced.
Another process is to first apply the solder needed to bond one set of leads and use a second stencil to apply the solder needed to bond the second set of leads. In practice, first the solder needed to bond the fine pitch leads is applied through a specifically designed stencil. The applied solder is then subjected to vapor phase reflow, which is a heating process, and a cleaning, so that it is solidified. Solder required for the standard lead bonding is then deposited on the printed circuit board. The components with standard leads are then mounted on the wiring board. The components with fine pitch leads are then mounted to the board. The process of mounting the components with fine pitch leads to the wiring board involves heating the board so that invariably the solder deposited thereon for the standard lead mounting is heated to the point where it starts to reliquefy, or reflow.
There are disadvantages associated with this method applying solder to the wiring board. Performing two stenciling steps substantially increases the complexity and the overall cost of the surface mounting process. For example, the solder initially applied for the fine pitch leads must be reflowed and cleaned so that when the second stencil is placed on the printed wiring board, the solder originally applied will not smear. Furthermore, during the heating step when the standard lead components are bonded, the solder initially applied for the fine pitch mounting is invariably heated, and may react with the metal forming the contact pads. This could adversely affect the composition, called the metalization, and integrity of the fine pitch lead bonding solder. Consequently, the joints formed by the solder may weaken to the point where lead break free from their contact pads.
Still another method of applying solder is to initially apply the correct amount of solder to each lead of a component, and then mount the component onto a wiring board. It is very costly to apply solder to the leads so that there is the correct amount on each lead. Moreover, this complicates surface mounting since a great deal of care must be taken to bond the fragile solder-coated leads so that each lead is properly bonded to a board contact pad, and that solder does not flow from the contact pads and from an electrical path between adjacent lead contact pad interfaces.